Maïté Blank
The Thermal Boundary Conductance (TBC) at the interface between metals and dielectrics is of key importance for heat transport in structures heterogeneous on a scale from 100 µm downwards. In this thesis the TBC of a large number of metal/dielectric couples was measured using Time-Domain Thermoreflectance (TDTR). Besides creating a large and coherently assessed number of metal dielectric pairs, a particular focus was laid on the effect of ultra-thin interlayers, as well as on the role of interfacial disorder and intermixing on the TBC. For those instances a framework to rationalize the observed phenomena was developed.
The large number of consistently evaluated TBC-values allowed comparing the predictive capacity of the widely used Diffuse Mismatch Model (DMM), with the more recently developed approach that combines the Maximum Transmission Limit (MTL) with an average transmission probability that characterizes phonons transmission across real interfaces. It is found that the MTL based prediction of TBC yields consistently good precision whatever substrate is used, while the prediction quality of the DMM decreases with increasing mismatch between the materials on either side of the interface. This work served as a reference to quantify the TBC changes observed in the rest of this work.
The influence of a coupling layer on the TBC of a given metal/dielectric couple was experimentally and theoretically assessed by using i) ultra-thin Cu interlayers (1.5 ¿ 25 nm) at the interface between Au and silicon, sapphire and diamond, as well as between Al and diamond, ii) ultra-thin interlayers of Mo and Ni at the interface between Ag or Au and diamond, and iii) intermixed interfaces in the Au-Si and Ti-Si couples with changes in the atomic order and chemical composition induced by various heat treatments. For case i) the TBC was observed to increase (Au-Cu-Si, Au-Cu-Al2O3, Au-Cu-diamond) or decrease (Al-Cu-diamond) monotonically for interlayer thicknesses up to 10 nm roughly, before reaching a plateau very close to the Cu-dielectric TBC value. The evolution of TBC was attributed to the finite electron-phonon coupling factor of the interlayer material for the Au-Cu-Si, Au-Cu-Al2O3, and Au-Cu-diamond cases. The apparent inconsistency between experimental evidence for the Al-Cu-diamond case and theory was rationalized by a variable coverage of the Cu-diamond interface by diffused aluminium evidenced by local chemical analysis.
The importance of the electron-phonon coupling factor was confirmed by the use of Ni and Mo interlayers with Au or Ag on diamond, for which the TBC increased monotonically over a much shorter range of interlayer thicknesses than the one observed for Cu, before reaching a plateau. The results were rationalized both with an analytical model developed earlier and a model based on the deviation between temperature of electrons and phonons, a so-called ¿Two Temperature model¿ (TTM) yielding good agreement.
In Au/Si samples, a change of the TBC was observed that can be attributed either to disorder or interdiffusion in the interfacial region. Investigations performed on the Ti-Si interface led to significantly decreasing TBC for increased duration of the heat treatment, apparently uncorrelated with the state of crystallization at the interface, but that may be due to interdiffusion.
EPFL2019
